On January 14 and March 4, 2026, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) convened the second and third Study Groups on Port Infrastructure for Offshore Wind Power. Across these sessions, FLOWRA’s European hearings, FLOWCON’s construction cycle modelling, and MLIT’s own facility scale studies surfaced a topic that had previously sat at the edge of the policy conversation: the temporary mooring of assembled floating foundations — wet storage.

The technical groundwork is now visible. MLIT has begun modelling storage water area requirements, including catenary mooring with drag anchors at water depths of 30–50 metres, and has set out two storage configurations: short-term in-port storage of one to two units, and over-winter offshore storage of up to ten units per location. FLOWRA has flagged the limits of drag anchors for pre-laid mooring and the density problem observed at Cromarty Firth in Scotland.

What is less visible — and what European practitioners have spent the last five years working through — is the gap between technical specification and deliverable infrastructure. This article examines that gap, drawing on a recent exchange with Will Rowley, Founder & CEO of Offshore Solutions Group, which is developing FLOW-Park wet storage facilities in Scotland, the Celtic Sea, Northwest France, and Western Ireland.

The topics covered in this article are part of a broader analysis examining floating offshore wind from a structural perspective — spanning technology, costs, regulations, and project viability. For a comprehensive overview, see the full guide below.
👉 Floating Offshore Wind in Japan｜A Structural Guide to Markets, Costs & Policy

Contents

1. What Japan’s Regulators Already Know

It is worth starting with what has already been established in the Japanese policy record. The conversation about wet storage in Japan is not absent — it has, in fact, advanced further than is widely recognised outside the immediate working groups.

MLIT’s Storage Water Area Modelling

MLIT’s facility scale study, presented at the third Study Group in March 2026, sets out three storage configurations for assembled semi-submersible steel floaters with a 7.0-metre draft:

In-port, fixed-anchor mooring: short-term storage of one to two units at 10-metre water depth, with a three-point taut-line mooring system
In-port, bottom-seated storage: short-term storage of one to two units at 10-metre water depth, used as a heavy-weather reserve configuration
Off-port, catenary drag-anchor mooring: over-winter storage of ten units at 30–50-metre water depth

For the off-port configuration, MLIT estimates an anchor circle radius of 232 to 292 metres per unit, depending on water depth. A 263-hectare water area requirement was identified for the Sea of Japan winter storage scenario in the 1GW reference case, where floater foundations manufactured in Western Japan would need to be towed and held until the May–October construction window opens.

FLOWRA’s European Findings

FLOWRA’s European hearing report, submitted in January 2026, identified three operational realities from European wet storage practice:

Pre-laid mooring is essential for shortening installation time. Suction or pile/drilled anchor types are required; drag anchors are unsuitable for pre-laid systems because their post-installation holding capacity cannot be guaranteed.
Water area capacity is constrained by safety distances, not surface area. Cromarty Firth in the UK has nominal capacity for around 70 units but, after applying collision-prevention spacing, the actual mooring capacity is approximately 40 units.
Distance from assembly and installation ports matters. Wet storage areas should be located such that towing to and from them does not require an intermediate refuge port in heavy weather.

The Drag Anchor Question

One specific tension is worth flagging. MLIT’s working assumption for the off-port over-winter configuration uses stockless drag anchors — the type FLOWRA has already cautioned is unsuitable for pre-laid systems in the European context. Whether Japan’s design assumptions converge on drag anchors (lower cost, larger area requirement, shorter installation lead time) or suction/pile anchors (higher cost, smaller area requirement, longer installation lead time) will materially change both the cost calculation and the permitting pathway.

2. What Five Years of UK Experience Adds

The technical specifications above describe what wet storage is supposed to look like on paper. The European practitioner experience describes what it takes to build one. The two views are complementary, but the second is harder to compress into a calculation.

The “Inverse Bell Shape” of Stakeholder Attention

According to Will Rowley, interest in wet storage among UK stakeholders has followed what he describes as an “inverse bell shape.” Initial attention was high — wet storage was identified by developer technical leadership teams as a critical-path item, and an industry Joint Industry Project (JIP) was launched with eleven developers. Interest then declined as developers focused on project economics, grid connections, and macro issues to move projects toward Final Investment Decision. Attention has since returned, often with a sense of urgency, as project teams have moved into delivery planning and re-encountered wet storage as a binding constraint.

This pattern is relevant to Japan because it suggests that the current period — when projects are not yet in detailed execution planning — is precisely the window in which wet storage tends to be deprioritised. By the time the issue returns to the developer agenda, the multi-year development timelines for actual storage facilities make late-stage solutions impractical.

Site Identification Is Harder Than It Looks

Offshore Solutions Group spent close to three years assessing more than 200 candidate locations across the UK before arriving at a short list of viable wet storage sites. Across other markets the company has assessed — France, Ireland, Italy, Australia, South Korea, Spain, Portugal, and Sweden — no “perfect” site has been identified. Most coastlines are subject to spatial conflicts with commercial anchorages, fisheries, environmental protections, or existing port traffic.

The implication for Japan is that the question is not “where could we put wet storage” but “which of the few viable locations can be secured, and on what timeline.” MLIT’s facility scale calculations assume that suitable water areas exist; the European experience suggests that establishing they do — and securing them — is itself a multi-year undertaking.

The Regulatory Interpretation Problem

This is the area where the gap between specification and delivery is widest. Wet storage typically requires pre-installing mooring infrastructure on the seabed in areas outside conventional port jurisdiction. In the UK and other European markets, none of the existing maritime, environmental, or trade legislation was drafted with floating wind logistics in mind. Seabed owners and regulators have therefore had to interpret existing legal instruments to accommodate the activity.

The Moray FLOW-Park is currently three years into a development plan that is expected to take six to seven years in total. A meaningful portion of that timeline is consumed by legal interpretation, stakeholder consultation, and consenting work — not by engineering. By the project’s account, governmental and political support at local, regional, and national levels has been necessary to navigate competing stakeholder positions.

Japan’s situation is structurally similar. Pre-installed anchors or temporary moorings outside conventional port areas would fall under existing maritime and seabed-use legislation — none of which was originally drafted with floating wind logistics in mind. To DeepWind’s knowledge, this question has not yet been tested in Japan in operational form.

A related layer that is often underweighted in early-stage studies is the role of marine insurance and classification society (e.g. DNV) acceptance. Maritime regulations and insurance frameworks frequently reference class approval or guidance, which means that securing classification acceptance for novel wet storage configurations is in practice a prerequisite for both consenting and commercial deployment. The European experience suggests this is one of the layers most consistently underestimated in scoping work.

3. Coordination Models: Why the UK and France Diverged

A separate but related issue is how multiple ports and storage facilities operate as a coherent system. This is where the European experience offers two contrasting models — and where Japan’s choice will matter.

The UK: Arms-Length Coordination

In the UK, ports have a poor record of effective coordination. Ports tend to regard each other as competitors rather than as nodes in an integrated logistics chain. Coordination on multi-port strategies has fallen largely on developers and their logistics partners, with limited contractual or operational backing from the ports themselves. The result has been delays in developers signing individual port contracts as they attempt to align highly integrated logistics chains across multiple sites.

The Scottish exception — where the UK Government, Scottish Government, and Crown Estate Scotland have provided support across multiple ports — has helped clarify roles between assembly, integration, and O&M functions in a way the market had not delivered on its own.

Northwest France: Integrated Coordination

In Northwest France, the regional government holds financial stakes in multiple ports and provides regional funding for port upgrades. The result is that ports are explicit about their market positions — assembly, integration, O&M, support — and openly coordinate multi-port strategies that allow developers and their T&I/EPCI partners to work through a single point of contact for multi-port delivery. This model has been welcomed by developers because it removes the burden of coordinating across competing port authorities.

The Port La Nouvelle case, already covered in DeepWind’s earlier analysis of the FLOWRA report, sits within this framework: the Occitanie Region invested approximately €490 million in infrastructure but contracted operations to SEMOP, a public-private joint venture that includes a private operator with experience across more than 40 ports.

For more on the Port Integrator model, see: Port Scaling and Integrated Operations — Deciphering the Latest FLOWRA Report

4. Japan’s Current Position — and the Timing Question

Set against the European experience, Japan’s current position can be characterised as follows: the technical concept is in active study, but the regulatory, locational, and coordination layers that European practitioners describe as the dominant constraint have not yet been substantively addressed.

The Project Timeline

FLOWCON’s modelling assumes that floating project formation will accelerate from FY2029, with port utilisation for fixed-bottom and floating projects overlapping for more than ten years thereafter. MLIT’s 1GW reference case assumes a two- to three-year construction cycle, depending on whether projects are sited on the Pacific or Sea of Japan side. Sea of Japan projects, in particular, depend on over-winter storage capacity to maintain project schedules across the November–April window in which offshore installation is largely impractical.

If wet storage facilities take five to seven years to develop — consistent with the Moray FLOW-Park reference timeline — then Japan would need to begin substantive site assessment, regulatory interpretation work, and stakeholder consultation in the 2026–2028 window in order for storage capacity to be available for the FY2029 onwards project pipeline.

The 263-Hectare Question

MLIT’s simulation for the Sea of Japan 1GW reference case identifies a winter storage water area requirement of up to 263 hectares, on the assumption that floater foundations are manufactured in Western Japan and towed to the project area. This is a calculation, not yet a sited facility. Identifying a 263-hectare water area that is sheltered enough to hold ten units through winter, located within practical towing range of an installation port, and free of conflicts with fisheries, commercial shipping, or environmental protections is the type of multi-year exercise the European experience suggests should not be underestimated.

For comparison, this is the work Offshore Solutions Group spent close to three years on in the UK, across more than 200 candidate locations.

5. Implications for Stakeholders

The wet storage question affects different stakeholders in different ways, and the responses required are not uniform.

For Developers

Project schedules — particularly for Sea of Japan projects with COD targets in the early 2030s — should account for the possibility that wet storage capacity may not be available on the timeline currently assumed in MLIT’s reference scenarios. This is not a reason to delay project formation, but it is a reason to engage early with the question of how individual projects will manage over-winter holding requirements if shared infrastructure is not yet in place.

For Regulators and Policymakers

The European experience indicates that regulatory interpretation work — clarifying which existing legal instruments apply to pre-installed mooring infrastructure outside port jurisdiction, and how — is a multi-year undertaking that needs to begin before commercial demand crystallises. Waiting for the first project to formally request seabed permission for a wet storage facility is likely to be too late.

For Port Authorities

The contrast between the UK and Northwest France suggests that the coordination model adopted now will shape execution outcomes for the next two decades. Japan’s port system has historically operated with strong coordination at the national policy level but limited cross-port operational integration. Whether that translates into a UK-style arms-length pattern or something closer to the French integrated model is a choice that has not yet been made explicit.

From Specification to Delivery

The framing that Japan “has not yet defined wet storage” is no longer accurate. MLIT, FLOWRA, and FLOWCON have collectively moved the topic from an unspecified concept to a partially specified set of design assumptions. That is meaningful progress.

The harder question is whether the gap between technical specification and deliverable infrastructure can be closed within the project timeline that the same agencies have set out. The European practitioner experience suggests that the parts of the work that do not appear in calculation tables — site identification, regulatory interpretation, stakeholder coordination, port-to-port operational integration — are the parts that consume the most time and shape the timeline most decisively.

For floating wind projects targeting CODs in the early 2030s, the window for proactive work on these layers is the next two to three years. The technical concept is now visible. The implementation question — who acts on which layer, and when — remains open.

[References]
This article draws on materials submitted at the second and third Study Groups on Port Infrastructure for Offshore Wind Power held on January 14 and March 4, 2026:

FLOWRA: Report on Hearing Results in Europe
FLOWCON: Construction Cycle and Challenges in In-Port Turbine Installation
MLIT: Conditions for Examining Facility Scale (Draft)
MLIT: Study of Port Facility Scale

The article also draws on a written exchange with Will Rowley, Founder & CEO of Offshore Solutions Group, which is developing FLOW-Park wet storage facilities in Scotland, the Celtic Sea, Northwest France, and Western Ireland. With thanks for the practitioner perspective shared in that exchange. The analysis and conclusions remain those of DeepWind.